The invention relates generally to the scheduling of run-time measurements and signalling reception in a cellular radio system. Especially the invention relates to the optimization of measurement and reception schedules in a situation where a mobile terminal is able to operate on several frequency bands and perform handovers between the frequency bands.
At the time of filing of this patent application a decision has been made to accept WCDMA (Wideband Code Division Multiple Access) as one of the multiple access schemes of an universal third generation cellular radio system known as the UMTS (Universal Mobile Telecommunication System), where a terrestrial base station subsystem is known as the UTRA (UMTS Terrestrial Radio Access). In CDMA, the transmission on a certain channel is generally continuous in contrast to TDMA systems (Time Division Multiple Access) where each channel reserves a certain cyclically repeating time slot. However, both in CDMA and TDMA it is customary to arrange the transmission into consecutive frames of constant duration.
In order for a mobile terminal to fully exploit the available cellular resources it will be very advantageous if it can communicate with both third and second generation base stations and move from the cell of one to the cell of another according to the momentary availability and pricing of services at a reasonable connection quality. In order to continuously look for the optimal base station to communicate with, a mobile terminal must perform measurements that reveal the signal strength it is able to receive from each candidate base station. Additionally the terminal must receive certain signalling messages from the candidate base stations in order to plan ahead for a cell reselection or a handover. It has been widely recognised that if a terminal is currently communicating with a CDMA base station, it needs some short time intervals during which the otherwise continuous downlink CDMA transmission is interrupted for the measurements and signalling reception to be possible.
Recently it has been shown that it is advantageous to interrupt also the uplink CDMA transmission for the duration of measurements and signalling reception. We will briefly describe the reasons behind this observation by reference to FIG. 1, where the GSM1800 system (Global System for Mobile telecommunications at 1800 MHz; also known as the DCS1800 or Digital Cellular System at 1800 MHz) is considered as an exemplary cellular radio system of the second generation. The uplink frequencies of the GSM1800 system lie between 1710 and 1785 MHz and the corresponding downlink frequencies between 1805 and 1880 MHz. From the downlink frequency range upwards there is a narrow range for DECT frequencies (Digital European Cordless Telephone) and another narrow range for UTRA TDD frequencies (Time Division Duplex). The UTRA FDD uplink frequencies (Frequency Division Duplex) lie between 1920 and 1980 MHz and the corresponding downlink frequencies between 2110 and 2180 MHz. Therebetween is also another relatively narrow range for UTRA TDD frequencies from 2010 to 2025 MHz.
If a mobile terminal is considering a handover from an UTRA FDD cell to a GSM1800 cell, it should be able to receive and decode signalling messages on the GSM1800 downlink frequency range. The UTRA FDD uplink frequency range is so close to the GSM1800 downlink frequency range that a simultaneous uplink transmission on the former is very likely to cause a RF leakage through the duplex filter of the terminal to the receiver chain, thus interfering with or even disabling any measurements or reception for decoding. There are potential hardware solutions to this problem, such as using a different antenna for different frequency ranges and very high quality duplex filters, but they typically require considerable complication of the terminal structure and are therefore undesirable for the manufacturing point of view. It is much easier to arrange for suitable interruptions also in the uplink transmission, especially if such interruptions have already been specified for the corresponding downlink.
The accepted form of interrupting a CDMA transmission in an UTRA system for measurements and signalling reception is known as the slotted mode. It means generally that a predetermined part of a certain frame period will be left empty with no ongoing transmission. The frame including the information contents destined for transmission during such a frame period will be transmitted during the remaining part of the frame period in a compressed form, using for example a slightly higher transmission power. Three types of slotted mode have been suggested for use; additionally we may regard a combination of two of them as a fourth type. FIG. 2 illustrates the different types of slotted mode in a coordinate system where the horizontal axis represents time divided into frame periods (e.g. 10 ms) and the vertical axis represents transmission power in some arbitrary units. Frame 201 illustrates an idle period at the end of the frame period, frame 202 illustrates an idle period in the middle of the frame period and frame 203 illustrates an idle period at the beginning of the frame period. Mutually consecutive frames 204 and 205 are of the first and third types described above, whereby it is immediately obvious that there will be a relatively long idle period bridging the separation line between these consecutive frame periods. The xe2x80x9cdouble lengthxe2x80x9d idle period may be regarded as a fourth type of slotted mode, even if it is actually just a suitable coordinated arrangement of slotted modes of the first and third type.
In principle it would be possible to have a fixed allocation of simultaneous slotted frames for all mobile terminals in the cell of a CDMA base station. However, due to the large number of potential connections simultaneously active such a common slotted mode is not advisable, because it would cause large jumps in the amount of emitted transmission power. In practice the base station will compose a time schedule for the use of slotted mode in its cell and use terminal-specific signalling to allocate a unique or nearly unique pattern of slotted frames to each terminal so that the overall effect of the slotted frames to the average transmission power within the cell will be negligible.
Next we will describe briefly the known arrangement of channels which a GSM1800 base station will use for transmitting those common channel messages which the mobile terminal operating in a nearby cell should receive in order to prepare for a cell reselection or a handover to the cell in question. A complete description of the GSM1800 common channels (which follow the arrangement of the corresponding channels in the conventional 900-MHz GSM system) is available to the public from the GSM specifications published by the ETSI (European Telecommunication Standards Institute) and e.g. from the book Michel Mouly, Marie-Bernadette Pautet: The GSM System for Mobile Communications, published by the authors, ISBN 2-9507190-0-7, Palaiseau 1992. In the following description we will emphasize the timing aspects, because these are of importance to the present invention.
Each GSM base station will regularly transmit, on a certain common channel frequency, so-called FCCH and SCH messages (Frequency Correction CHannel; Synchronisation CHannel). The transmission schedules of all GSM channels have been determined in relation to the concept of a frame, which contains 8 consecutive time slots or BPs (Burst Periods) each having the length of 15/26 ms (approximately 0.577 ms). On said common channel frequency we may take a period of 51 frames and designate the frames therein from 0 to 50; in such arrangement the first time slot of the 0th, 10th, 20th, 30th and 40th frame will contain an FCCH message and the first time slot of the 1st, 11th, 21th, 31th and 41th frame will contain an SCH message. In other words we may say that a GSM base station will transmit on said common channel frequency FCCH messages with regular intervals so that four consecutive intervals will be of approximately 46.154 milliseconds and the fifth interval after them will be approximately 50.769 milliseconds, and an SCH message will follow each FCCH message approximately 4.615 milliseconds later.
The frame duration of the UMTS has been defined to be 10 ms. It has been noted that the relation of the FCCH and SCH schedules of GSM to the frame timing of UMTS is such that no periodicity of relatively short length will occur. The advantage of such non-periodicity is that when an UTRA base station will allocate certain frames for slotted mode, it is inevitable that sooner or later the measurement interval left free by a slotted frame will coincide with the transmission moment of an FCCH or SCH message from a nearby GSM base station.
The disadvantage of the above-explained prior art system is that in order for the mobile terminal to find, receive and decode a sufficient number of FCCH and/or SCH messages from the nearby GSM base stations a relatively large number of slotted frames will be required. Not only is the probability of error-free reception of a slot ted-mode compressed frame lower than that of a regular frame; we must also take into account the higher power level needed to transmit the compressed frame. It is known that the transmission power level in each CDMA connection affects the interference level experienced by other simulteneous connections with a direct proportionality, in the same cell as well as in the neighboring cells. All in all it has been suggested that the slotted mode in its known form might eat up to 15 to 20 per cent of the total WCDMA system capacity.
It is therefore an object of the present invention to provide a method and arrangement for enabling the necessary measurement and reception functions during an active CDMA connection with only a minor effect on the overall system capacity.
The objects of the invention are achieved by providing a CDMA base station with knowledge about the relevant transmission schedules of the nearby other base stations so that the slotted frames may be allocated in optimal way.
The method according to the invention is characterised in that it comprises the steps of
establishing knowledge that indicates at least one future occurrence of signalling external to a first communication connection,
identifying, on the basis of said established knowledge, a future frame associated with the first communication connection that will coincide in time with said future occurrence of signalling external to the first communication connection and
defining the identified future frame as a frame allocated for measurement and reception of signalling external to the first communication connection.
The invention also applies to a base station subsystem having as its characteristic features
means for establishing knowledge that indicates at least one future occurrence of signalling external to a first communication connection,
means for identifying, on the basis of said established knowledge, a future frame associated with the first communication connection that will coincide in time with said future occurrence of signalling external to the first communication connection and
means for defining the identified future frame as a frame allocated for measurement and reception of signalling external to the first communication connection, and for communication such a definition to the mobile terminal.
The large amount of wasted capacity in a prior art arrangement is caused by the fact that the CDMA base stations allocate slotted frames to mobile terminals without any knowledge about the transmission schedules of the nearby other base stations. Consequently the mobile terminals will spend a relatively large number of measurement intervals by bootlessly looking for FCCH and/or SCH messages that simply are not on the air during a certain measurement interval. According to the invention a CDMA base station, or more generally a base station employing a substantially continuous transmission mode, will know beforehand, when a nearby other base station will transmit such a message that a mobile terminal should be able to receive. The CDMA base station will then allocate a slotted frame so that it coincides with the known transmission instant of the message in question.
There are several ways for providing a base station (or some other network device that is responsible for the slotted mode allocations) with the knowledge about the transmission schedules of nearby other base stations. In many cases a CDMA base station and the other base station will be installed at the same site, so a short cable from one equipment rack to another is enough to convey the required information. Even if the base stations are more distant from each other such a direct connection on the base station level is possible. An alternative embodiment requires that a base station controller or some other network element that is farther away from the base station towards a mobile switching centre or similar central installation collects the timing information that describes the operation of a number of base stations and communicates it to another base station controller, which then distributes the information to its own base stations. A further embodiment does not require explicit transmission of timing information at all: a CDMA base station may have a radio receiver of its own for continuously or regularly monitoring the FCCH and SCH transmissions of nearby other base stations so that it will find out their regular schedules.
By arranging the slotted mode allocations to the terminals according to unique or nearly unique allocation patterns, and by ensuring that at least a considerable number of the allocated slotted frames coincide with simultaneous signalling messages from the nearby base stations a CDMA base station will be able to remarkably diminish the amount of capacity wasted due to the use of slotted mode.